I have an 1988 sammi, When accellerating hard there is a pronounce whine/rumble from what sound like the Transmission.  Back of the throtlle and it diminishes, but doesn't go away.  Push in the clutch and it's gone so I'm pretty sure it's the drivetrain.  Could it be the TC?

It might be the bearings in the transmission.  Does the noise increase when you lightly push the shifter to the right or left while in gear?

I was having the same problem and it was my transmission bearings that were going out

Pushing on the shift lever doesn't appear to affect the noise.  Interestingly the "whine/growl" is almost non-existent sometimes.  Just when I begin to think its the TC or rear diff it comes back. Definitely more noticable under load and completely gone when you push in the clutch at any speed.

It could be the Through Out Bearing giving up the Ghost.

I had 2 different drive line failures;
1st was a growl that I thought was the rear Diff going South, turned out to be a Right Rear Axle bearing
2nd, had a what I thought was the Tranny going bad, it was a rear output bearing in the T-case that I found when I did the     4.9 gear swap. Lucky I had ordered all the new bearings to install at the same time.

It could be the Through Out Bearing giving up the Ghost.

It could be - but if it is - that means the clutch is incorrectly adjusted.

Under normal circumstances the throw out bearing should be sitting there doing nothing, not even rotating - until you depress the clutch pedal, bringing it into contact with the fingers on the clutch plate.

If a defective throw out bearing is going to make noise, it'll usually do it when the pedal is depressed, not when it's released.

It is possible, if the clutch has been incorrectly adjusted, for the throw out bearing to be in contact with the clutch plate, and to go silent when you push the pedal, but, I don't think it's going to be a "pronounced whine/rumble" that varies "power on/power off".

Put the transfer case in neutral and run it through the gears - if the noise is still present, it'll be either in the transmission or transfer case - if it's not, it'll be the axle or the transfer case ouput

Thanks fordem!  I was looking for a way to isolate the problem.  You just gave me one.